288 GEOLCKJY OF THE EUREKA DISTRICT. 



In applying- this liypothe.sis of differentiation to molten masses the 

 question naturally arises, What would have resulted at Eureka if the slow 

 processes of differentiation going on in a magma l^efore final crystallization 

 had either terminated earlier or progressed still further! On the one 

 hand, supposing a separation less complete than that at Eureka, a stage in 

 the development would be reached when a feldspathic magma would form 

 consisting of hornblende-mica-andesite or dacite, or more probably both, 

 followed by pyroxene-andesite without the interpolation of any body of 

 rhyolite. On the other hand, if the segregation of feldspathic magma had 

 gone on more completely than we find it at Eureka, there might have been 

 formed the same sequence of feldspathic lavas, only with a much larger 

 extravasation of rhyolite, in turn followed by basalt, without the inter- 

 polation of pyroxene-andesite. Again, the eai'liest rock might have been 

 hornblende-mica-andesite of the feldspathic magma, succeeded rapidly by 

 pyroxene-andesite. If this series of lavas had been followed by a cessa- 

 tion of volcanic energy and a long interval of rest, and then by a renewal 

 of activity, the final product, after a still further separation of the magma, 

 would result in the extravasation of rhyolite and basalt. This latter 

 sequence of lavas gives the order of succession so frequently met with 

 throughout the Great Basin. At Eureka, as already described, no long- 

 time interval, geologically speaking, is recognized between the andesites 

 and rhyolites, while the dacites and rhyolites frequently present the appear- 

 ance of continuous flows. 



In considering these phenomena it is important to bear in mind the 

 facts so frequently observed elsewhere in the Great Basin, that a crystalline 

 lava derived from a feldspathic magma of intermediate composition is, in 

 many instances, as shown by Richthofeu and King, followed by a pyroxene 

 lava, and the latter is almost invariably of intermediate composition; a lava 

 still more acid by one correspondingly basic, and the extreme acid type by 

 the extreme basic type. A rhyolite may be followed by pyroxenic lavas 

 varying in composition, but the writer knows no instance in the Great 

 Basin where a rhyolite is succeeded by a more basic feldspathic rock, nor 

 where a basalt is followed by a less basic pyroxenic lava. 



